1. Field of the Invention
CALCIUM CARBONATE SCALE INHIBITION--The present invention relates to the use of monofluorophosphate salts, especially sodium monofluorophosphate (Na.sub.2 PO.sub.3 F), in a method of inhibiting the formation of calcium carbonate (CaCO.sub.3) scale deposits on metallic surfaces of water-carrying systems. Generally, calcium carbonate scale deposits are incrustation coatings which accumulate on the metallic surfaces of a water-carrying system through a number of different causes.
Various industrial and commercial water-carrying systems are subject to calcium carbonate scale formation problems. Calcium carbonate scale is of particular concern in heat exchange systems employing water, such as, for example, boiler systems, and once-through and open recirculating water cooling systems.
The water employed in these systems ordinarily will contain a number of dissolved salts, and the alkaline earth metal cation calcium is usually prevalent, as is the anion carbonate. The combination product of calcium cation and carbonate anion will precipitate from the water in which they are carried to form scale deposits when the concentration of the anion and cation comprising the reaction product, i. e., calcium carbonate, exceeds the solubility of the reaction product itself. Thus, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction product, a solid phase of calcium carbonate will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentrations of the constituent ions are no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for the reaction product calcium carbonate. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system.
For boiler systems and similar heat exchange systems including cooling towers, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. As a result, the solubility of the scale-forming calcium carbonate salt reaction product is first exceeded in this thin film, and crystallization of calcium carbonate scale results directly on the heating or heat exchange surface.
In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate, water and carbon dioxide under the influence of heat. For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes calcium carbonate scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated two, four, or six times.
The formation of calcium carbonate scale deposits poses a serious problem in a number of regards. The calcium carbonate scale which is formed possesses a low degree of heat conductivity. Thus, a calcium carbonate scale deposit is essentially an insulating layer imposed across the path of heat travel from whatever source to the water of the system. In the case of a boiler system, the retarded heat transfer causes a loss in boiler efficiency. Increased input of heat to compensate for this loss results in overheating of the boiler metal and consequent tube failures. In addition to this problem, calcium carbonate scale formation facilitates corrosive processes, and a substantial calcium carbonate scale deposit will interfere materially with fluid flow. Consequently, calcium carbonate scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
STABILIZATION OF SOLUBLE MANGANESE AND ITS REACTION PRODUCTS--The present invention further relates to the use of monofluorophosphate salts, especially sodium monofluorophosphate (Na.sub.2 PO.sub.3 F), to stabilize soluble manganese ion and its reaction products in desirable forms and reduced particle sizes. Manganous ions are often found in well waters while cooling waters contain primarily the manganic species. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of manganese and iron can collect on metal surfaces and accelerate corrosion and reduce heat transfer.
Oxidation leads to precipitation of dark brown or black hydrous oxides or hydroxides of the higher oxidation states of manganese which are very insoluble. When these precipitates remain suspended in the water, they cause objectionable discoloration known as "black water"; when they settle out, black deposits form which can block lines, or act as catalysts causing further manganese deposition. These deposits are very deleterious in textile and laundry operations as they interfere with dying processes and leave spots which are difficult to remove. They appear to increase the corrosion of copper. They are also troublesome in municipal water distribution systems where their presence makes it extremely difficult to maintain a chlorine residual.
The monofluorophosphate salts, especially sodium monofluorophosphate (Na.sub.2 PO.sub.3 F), when used in accordance with the method of the present invention, can keep the reaction products of manganese described above in colloidal/fine dispersed form rather than the normal flocculant, adherent species. The manganese thus remains soluble so that it will not form particles which will precipitate out of solution and form scale.
STABILIZATION OF SOLUBLE IRON AND ITS REACTION PRODUCTS--The present invention still further relates to the use of monofluorophosphate salts, especially sodium monofluorophosphate (Na.sub.2 PO.sub.3 F), to stabilize soluble iron ion and its reaction products in desirable forms and reduced particle sizes. Ferrous and ferric ions are often found in well waters while cooling waters contain primarily the ferric species. Anionic species of carbonate, bicarbonate, sulfite, fluoride, chloride, sulfate, and so forth, and dissolved oxygen may be present in both waters. Oxygen reaction products of iron can collect on metal surfaces and accelerate corrosion and reduce heat transfer.
Oxidation leads to precipitation of brown or red oxides of the higher oxidation states of iron which are insoluble. When these precipitates remain suspended in the water, they cause objectionable discoloration known as "red water"; when they settle out, red deposits form which can block lines, or act as catalysts causing further iron reaction product deposition. These deposits are very deleterious in textile and laundry operations as they interfere with dying processes and leave spots which are difficult to remove. They are also troublesome in municipal water distribution systems where their presence makes it extremely difficult to maintain a chlorine residual.
The monofluorophosphate salts, especially sodium monofluorophosphate (Na.sub.2 PO.sub.3 F), when used in accordance with the method of the present invention, can keep the reaction products of iron described above in colloidal/fine dispersed form rather than the normal flocculant, adherent species. The iron thus remains soluble so that it will not form particles which will precipitate out of solution and form scale.
2. Brief Description of the Prior Art
Early efforts to reduce scale formation in water-carrying systems employed compounds such as tannins, modified lignins, algins, and other similar materials. Chelating or sequestering agents have also been employed to prevent precipitation or crystallization of scale-forming calcium carbonate. Another type of agent which has been actively explored heretofore as a calcium carbonate scale inhibiting material is the threshold active inhibitor. Such materials are effective as scale inhibitors in amounts considerably less than that stoichiometrically required, and this amount is termed the threshold amount. Inorganic polyphosphates have long been used as such threshold active inhibitors. For examples of such materials, see Fink--U.S. Pat. No. 2,358,222; Hatch--U.S. Pat. No. 2,539,305; and Ralston U.S. Pat. No. 3,434,969. Certain water soluble polymers, including groups derived from acrylamide and acrylic acid have been used to condition water containing scale-forming calcium cargonate. For example, see U.S. Pat. No. 2,783,200; 3,514,476; 2,980,610; 3,285,886; 3,463,730; 3,518,204; 3,928,196; 3,965,027and 4,936,987.
Methods which have been used heretofore to remove manganese include those whereby the manganous ion is oxidized to insoluble higher oxides, hydrous oxides, or hydroxides, which precipitate and may be removed by coagulation and settling, filtration, or both. The oxidation has also been effected by raising the pH of the water to 8 or higher where naturally occurring dissolved oxygen or mechanical aeration brings about oxidation, or by the use of chlorine or permanganate. All of these methods, however, suffer from obvious disadvantages which limit their usefulness and effectiveness. For example, the use of a high pH to facilitate oxidation by dissolved oxygen is expensive and tends to cause scale deposition. Chlorine is only slightly more active than dissolved oxygen for oxidation of manganese and also requires pH elevation. Permanganate is expensive and imparts to the water an intense color that may be unacceptable.
One method for removing the manganese by precipitation and removal involves the addition of a salt of iron, copper, or cobalt and any compound yielding bisulfite ions in solution to the manganese-containing water. See Hatch--U.S. Pat. No. 3,349,031.
Soluble manganese ion and its reaction products have been stabilized in water systems using carboxylic acid/sulphonic acid copolymers. See Ralston--U.S. Pat. No. 4,552,665.
Moran et al., U.S. Pat. No. 4,613,450 discloses corrosion inhibitors comprising members of the fluorophosphate family, including sodium monofluorophosphate. However, these are said to be useful for protecting metallic surfaces of installations and devices using water as energetic or thermic fluid, i.e., for heating and cooling. The only metals which are mentioned are iron and its alloys, particularly galvanized steel, copper and its alloys, and aluminum and its alloys.
Sodium monofluorophosphate is the most widely accepted dentifrice additive to reduce dental decay. In aqueous solution as well as in a paste, it has been reported to be effective in treating sensitive teeth.